Hariri made the claim in response to a question about alleged assassination plots against the leaders by Syrian President Bashar al-Assad's brother-in-law and head of intelligence Assef Shawqat.
"We have intelligence about this and we are following it up," he told reporters.
"The intelligence is correct and our security services are working on it.
"There is cooperation between Lebanese security services and Arab security services to avoid such assassinations," he said, without specifying which countries were helping in the investigations.
Asked about Hariri's comments, an official at the Beirut office of the Western-backed Siniora said: "It is true and we have been informed about it."
Several anti-Syrian figures have been assassinated in Lebanon since Hariri's own father and former premier Rafiq was killed in a February 2005 bombing.
An initial UN inquiry into that killing implicated Damascus and its allies in Lebanon, where four pro-Syrian security chiefs were arrested in late 2005. Syria has vehemently denied any involvement.
Saad has been at the vanguard of Lebanese efforts to set up a UN-backed tribunal to judge those responsible for his father's death, saying earlier this month that "what is happening in Lebanon today is a destabilizing coup."
Political tensions are running high as rival pro and anti-Syrian factions in Lebanon have failed to agree on a consensus candidate to succeed President Emile Lahoud, a Damascus protege whose term in office ends next month.
In the latest political assassination, six people were killed last month in a Beirut car bombing, including anti-Syrian MP Antoine Ghanem, just days before parliament was due to meet to choose a new president.
MPs are now set to meet on November 12 in a last-ditch bid to elect a head of state, and avoid further political turmoil amid fears that the impasse over the presidency could lead to two rival governments.
Hariri said his talks with Egyptian President Hosni Mubarak in Cairo focused on the vote and "the interference of certain states to prevent the holding of these elections."
He said Mubarak had promised to help prevent "any interference in the Lebanese presidential election or anything that might prejudice the stability of Lebanon."
He noted that the most recent political assassination came a few days after parliament speaker Nabih Berri, a member of the pro-Syrian opposition, appealed for dialogue.
"We feel that this is a way of undermining Lebanese dialogue and the Lebanese consensus," he said.
Hariri earlier this month appealed for increased international pressure to set up a court to try suspects in the assassination of his billionaire father.
"We asked for a harder position from the United Nations in the face of those assassinations," Hariri said told UN chief Ban Ki-moon in New York on October 9.
Tags: Beirut, Lebanese, Lebanon, Saad Hariri, Siniora
